Satellite Master Antenna Television (SMATV) systems currently receive analog television signals from satellites and distribute these analog television signals by way of a cable to a plurality of users. For example, SMATV systems may be used in high rise apartment buildings. It is desirable to similarly distribute digital television signals received from satellites.
While it is possible to directly distribute the L-band signals from a digital satellite service to receivers, a direct distribution arrangement would require the addition of a costly and complex signal distribution system. An alternative to such a direct signal distribution system is to re-modulate each QPSK satellite signal to a QAM signal of the existing SMATV system, and to then supply the resulting QAM signal into a corresponding unused CATV channel of the SMATV system. This type of re-modulation is usually referred to as transmodulation. In transmodulation, a transmodulator converts the satellite signal to a QAM CATV signal by tuning to the QPSK satellite signal, by transmodulating the received QPSK satellite signal into a QAM signal, and by supplying the QAM signal over a corresponding unused CATV channel.
In addition, it is noted that digital satellite signals contain both the picture data that are used to generate television pictures and also service data that include the QPSK carrier frequency of the corresponding satellite channel as well as the data that must be processed in order to generate the video and audio for that satellite channel. Because the relationship between the CATV frequencies carrying the QAM transmodulated signals and the satellite frequencies of the original QPSK signals may vary from SMATV system to SMATV system, the service data contained in digital satellite signals is of no use to QAM set top boxes in tuning to the signals on the SMATV cable.
Therefore, another alternative might be to replace the satellite service data with cable TV service data that the set top box of the SMATV system could then recognize in order to ensure proper tuning. Unfortunately, this alternative requires the purchase, installation, and maintenance of very expensive equipment at each point of distribution.
A better alternative would be to store a frequency translation table in non-volatile memory of a set top box. This translation table would, therefore, contain the relationship between each satellite channel frequency indicated by the service data in the satellite signals and the corresponding unused cable channel frequency of the transmodulated signal. Accordingly, when a user selects a satellite channel, the set top box looks up the corresponding cable channel frequency in the translation table and tunes the CATV tuner to the proper cable channel frequency corresponding to the satellite frequency of the selected satellite channel.
A problem with this approach is the need for an accurate translation table to be loaded into the non-volatile memory of the set top box prior to or at the time of installation. However, as discussed above, there is no single relationship between satellite frequencies and the cable frequencies to which the satellite frequencies are transmodulated by the SMATV system that covers all SMATV systems. Therefore, loading such a table at the factory is impractical. Instead, the table could be manually loaded at the time of installation. Unfortunately, manual loading of the table at the time of installation would be tedious and time consuming and, as a result, would likely lead to entry errors.
The present invention permits automatic loading of a translation table at installation of a set top box. In order to implement this automatic loading, the set top box is arranged to scan through the entire CATV frequency spectrum in order to find all possible transmodulated QAM signals. As each QAM signal is found, the satellite frequency contained in the satellite service data of the QAM signal is read and is loaded into the translation table in association with the cable frequency at which the QAM signal was found. Accordingly, when a user desires to tune to a particular program in one embodiment of the present invention, the user selects the appropriate satellite channel, the satellite service data corresponding to the selected satellite channel is read, the satellite frequency contained in this satellite service data is used to read the corresponding cable frequency from the translation table, and the CATV tuner is tuned to that cable frequency.